1. Technical Field
The present disclosure relates to a bi-directional coupler, and more particularly to a bi-directional coupler using an improved terminal circuit.
2. Description of Related Art
A directional coupler is a radio frequency (RF) component/device which provides three communication ports, an input port, an output port and a coupled port. RF signals enter the directional coupler via the input port, where only a small portion thereof is output via the coupled port while the remaining is output via the output port.
FIG. 5 shows a schematic view of one such bi-directional coupler 100. The bi-directional 100 provides 4 communication ports: an input port 110, an output port 120, a coupled port 130 and an isolated port 140. The bi-directional coupler 100 further provides two terminal circuits 150 and 160 which respectively comprise terminal resistors 155 and 165, each connecting to a ground. The resistance value of each of the resistors 155 and 165 is 50 ohm (Ω), in one example.
FIG. 6 shows a schematic view of an equivalent circuit of the bi-directional coupler 100 shown in FIG. 1. When a transmitter (TX) 170 delivers RF signals to the bi-directional coupler 100 via the input port 110, a large portion of the RF signals are forwarded to an antenna 180 via the output port 120. Meanwhile, a lesser portion of the RF signals are transmitted to the coupled port 130 while no RF signal is output via the isolated port 140 (in an ideal manner). On the contrary, when RF signals are delivered to the bi-directional coupler 100 via the output port 120, a large portion of the RF signals are forwarded to the transmitter 170 via the input port 110. Meanwhile, a lesser portion of the RF signals is transmitted to the isolated port 140 while no RF signal is output via the coupled port 130 (in an ideal manner).
However, accuracy of termination values of a terminal resistor may be affected due to manufacturing processes and temperature variations and parasitical effects of parasitical capacitors, thereby increasing return loss and diminishing isolation of a coupler. In other words, referring to FIG. 2, the terminal resistor 165 of the terminal circuit 160 involves an allowed tolerance range so that signal transmission may be impeded due to variations of a resistance value of the terminal circuit 160.